Built-Up Posts vs. Solid Posts: A Quick Comparison
This past week, we took a look at a deck built many years ago by some unknown contractors (who they are or were is not clear to us, at least). For some reason the particular contractor who built this deck decided to use a combination of three sets of 2×6 boards together for each post instead of simply using 6×6 lumber. We assume they did this to cut a corner and save a dollar, but we aren’t really sure because it was built so long ago.
The original contractors aren’t here, so we can’t ask them why they did this, and it’s not very clear because it doesn’t really help to build something like this. It just might save a little bit of money if they have a bundle of excess materials left over from another project, for example. But it’s not the sort of thing that somebody would typically do in a professional or new type of construction.
This past week we also talked about the building code and some of the rules that the building code has about this type of construction. The building code recommends using solid posts instead of three individual boards nailed together. Particularly, when laminated board elements like this are going to be used together and in a built-up sort of compilation, the building code, at least, has specific rules about how they must be joined and installed.
From a structural perspective, common nails are insufficient, for several reasons, for laminating or joining several boards like this in a sandwich together. Common Nails might work okay and different types of arrangements or uses, but particularly where boards are assembled like this and need to resist lateral forces that otherwise separate the individual members of the sandwich of boards, common nails are generally inadequate to keep the individual pieces of lumber bound together. Common nails have a very low resistance to forces that cause the nails to withdrawal or slide out from the boards.
In most cases, typical common Nails will offer better sheer resistance than the typical construction screw, or at least the common phosphate or magnesium type drywall screws. Those screws are generally associated with relatively low shear resistance. However, in the case of common nails, we share resistance may be rated even less than some of the structural screws used for debt construction. Those screws are extremely expensive, compared to most average fasteners, and they have much different characteristics and ratings than the typical almost similar to looking counterparts.
As you look down the length of the post, you can see that something with significant resistance that would bind these boards together, such as carriage bolts, has not been used. The only thing we can find here is typical common nails. Also, as you can see in the next picture below there isn’t a resistive type of connection to the ground. The building code does require that the post are set on top of the footings.
The connection to those individual footings should either be made with an aluminum type of offsetting base plate which holds the post just above the ground or at least a u-shape type assembly that wraps around at least two sides of the post. In some cases the latter option might hold the post closer to the ground. However, there are structural connectors available that both connect the sides of a post to the base plate and also have an offset to hold the base plate roughly three quarters of an inch above the adjacent grade or substrate footing.
In the photograph, up at the top of the page, and in the next photograph below, you can clearly see that these boards are separating from one another. Over time, with the weight of the deck above imposed upon each of these individual posts, and the typical types of micro-movements and shifts that happen with the wind pressures and lateral forces, these posts have had a bit of movement that’s affected them. You can see that at each iteration of that movement, since the individual boards are not properly fastened or adhere to one another, with a liquid applied adhesive, the joint between those ports has weakened.
Structural forces and types of tensile resistances are very interesting because there are synergies in building material and assemblies of those materials. For example, the rating of one type of building material resistance might be x, a variable just used for explanation. But 10 of those materials, particularly when joined together, may have a structural resistance way more than 10x. This type of hypothetical conjecture points out that together, building materials that are individually comparatively weak might be way stronger when joined in a unified resistance force. There was an old TV commercial that showed two elephants or two cars pulling a phone book apart where the phone book was connected by interlaced pages.
The pages stood up to an enormous amount of pressure but everyone knows it’s easy to tear a page from a phone book. It takes less than a pound of pressure. Together though when unified or interlaced in a way that creates friction between the individual elements, they have a much greater synergistic type of overall resistance. That sort of context applies to these posts as well.
Here in the pictures of the above and below, we also show additional views of the base of these posts where it touches directly to the concrete patio and in the adjacent garden planter. Particularly with untreated wood or wood that isn’t rated for underground installation, it’s never a good idea to have that wood in direct contact with dirt or a garden bed.
We recommend building and homeowners here in Washington DC use a contractor, like us, who understands and cares about quality, deck and patio longevity, snd simply doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios, you can give us a can or just drop us a line. We are happy to help with almost all steps of the deck and patio building and design process. Let us know about your ideas and talk to us if you have questions about possibilities. You can call us at (202) 774-9128. You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com
